Return Air Grille Sizes: 3 CFM Charts + Grille Size Calculator

In HVAC, we have to choose the right return air grille size. Everybody knows that an adequately sized grill will ensure smooth airflow (essential). However, not everybody knows what return grill size to choose.

What size return air grille do you need?

There are two ways how to figure out what return grille size you need:

1. Calculate the return grille size. We will show you how to properly estimate the grille size in CFM as well as in inches. We will use the 500 CFM face velocity and 70% grille free area to make the calculation. To help you out, we created a grille size CFM calculator that makes the calculation automatically (just insert airflow needed in CFM, and you will get a calculated grille size in square inches).
2. Check the return grille size CFM chart. The grille size chart tells you what grille size you will need for your HVAC system. Below, you will find 3 grille size CFM charts – 1st one for horizontal grilles, 2nd one for square grill, and the 3rd one for vertical grilles.

What happens if the grill size is not appropriately sized?

• Choosing a grille that is too small will basically suffocate your HVAC system. The airflow HVAC system needs will not be sufficient if we choose a return grille size that is too small. Small grills can increase the noise levels as well due to increased ‘sucking’ of air.
• Choosing a grille that is too big is less problematic. The HVAC system will have sufficient airflow but we will unnecessarily have to sacrifice additional space for the grille. This will also increase the overall cost of our HVAC system.

Below the 3 grille size CFM charts, we also solve 3 examples of adequate grille sizing to demonstrate what size grille you will need for your HVAC system.

Note: ‘Grille sizes’ are usually used to determine cold air return vent cover sizes.

Let’s first look at how we can calculate the grille size before we look at the calculator and the all-encompassing grille CFM charts:

How To Calculate Return Grille Size?

Let’s presume that our HVAC system requires 400 CFM airflow. The grille size has to be big enough (but not too big) to allow for 400 CFM airflow.

Now, to properly calculate the return grille size, we have to make these two assumptions:

1. Face velocity is 500 FPM (feet per minute). This is the speed at which the return grille sucks the air from your house into your ducts.
2. Free grille area is 70%. As we know, the grille is always bigger than the duct through which the airflow flows. That is because the grille has to be screwed on the wall; for this, it will need to overlook the duct, resulting in some grille free area loss. Some grilles have 50% free area, others 60%, 70%, 80%, or even 90%. The 70% is the standard average we use when accounting for the free grille area.

With these two concepts in mind, we can look at this return grille size formula:

Grille Area (Sq In) = Airflow (CFM) ÷ Face Velocity (FPM) ÷ Free Area (Percentage) × 144

In this equation, we see that we have the desired airflow (measured in CFM), the face velocity (we presume this is a standard 500 FPM), free area (we presume 70% free area), and the 144 factor that converts the grille size into square inches.

By using this equation, we can determine the size of the return grille we need for 400 CFM airflow like this:

Grille Area (400 CFM) = 400 CFM ÷ 500 FPM ÷ 0.7 × 144 = 164 Sq In

That means we will need a grill with at least 164 square inches of area to accommodate for a 400 CFM airflow.

10×10 grille, for example, has a 100 sq in area; it will be too small. 14×20 grille has a 280 sq in area; it will be too big. The closest grille with at least 164 square inches area are 18×10 grill with 180 sq in area and 6×30 grille with 180 sq in area.

This is how we select a grille size in inches based on the required minimum grille size in square inches.

To make this calculation much easier, you can use this calculator:

Return Grille Size Calculator

You just insert the airflow your HVAC system needs (in CFMs), and you will get the minimal area of the grille size you need:

Example: Try calculating the minimal grille area for 600 CFM. You just slide the calculator to ‘600’ and you get the calculate grille size ‘247 square inches’. This means you need a grille size with at least 247 sq in area.

How to match the grille area to the actual grille size in inches?

You can use the following return grille CFM chart. There are 3 charts, namely:

• Horizontal grille size CFM chart. Horizontal grilles have a longer width dimension than the height dimension.
• Square grille size CFM chart. Square grilles have the same width and height dimensions.
• Vertical grille size CFM chart. Vertical grilles have a shorter width dimension than the height dimension.

Let’s look at each of these CFM charts in turn:

Horizontal Grille Size CFM Chart

Example 1: Let’s say we are looking for a horizontal return grille for 500 CFM airflow. If you look at the chart above, you can see that the 10×30 grille allows for 525 CFM airflow. This is the best choice. You could also pick an 8×32 grille will 583 CFM airflow.

Square Grille Size CFM Chart

Example 2: Let’s saw we are looking for a square return grille that can handle 1,000 CFM airflow. The 20×20 grille allows only for 972 CFM airflow; it will be too small. The correct return grille size for 1,000 CFM is 22×22 grille.

Vertical Grille Size CFM Chart

Example 3: Let’s say we are looking for a vertical grille size that can handle 800 CFM. Checking the chart above, we see that the 24×14 grille (813 CFM) comes closest (but not under) 800 CFM.

With these charts, you can effectively look up the grille size you need. If you have any questions about these grille sizes or would like for us to calculate your grille size, you can use the comments below and we’ll try to help you out.